Rollup merge of rust-lang#137953 - RalfJung:simd-intrinsic-masks, r=WaffleLapkin

simd intrinsics with mask: accept unsigned integer masks, and fix some of the errors

It's not clear at all why the mask would have to be signed, it is anyway interpreted bitwise. The backend should just make sure that works no matter the surface-level type; our LLVM backend already does this correctly. The note of "the mask may be widened, which only has the correct behavior for signed integers" explains... nothing? Why can't the code do the widening correctly? If necessary, just cast to the signed type first...

Also while we are at it, fix the errors. For simd_masked_load/store, the errors talked about the "third argument" but they meant the first argument (the mask is the first argument there). They also used the wrong type for `expected_element`.

I have extremely low confidence in the GCC part of this PR.

See [discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/channel/257879-project-portable-simd/topic/On.20the.20sign.20of.20masks)

Author: ChrisDenton

src/intrinsic/simd.rs

@@ -447,9 +447,14 @@ pub fn generic_simd_intrinsic<'a, 'gcc, 'tcx>(
             m_len == v_len,
             InvalidMonomorphization::MismatchedLengths { span, name, m_len, v_len }
         );
+        // TODO: also support unsigned integers.
         match *m_elem_ty.kind() {
             ty::Int(_) => {}
-            _ => return_error!(InvalidMonomorphization::MaskType { span, name, ty: m_elem_ty }),
+            _ => return_error!(InvalidMonomorphization::MaskWrongElementType {
+                span,
+                name,
+                ty: m_elem_ty
+            }),
         }
         return Ok(bx.vector_select(args[0].immediate(), args[1].immediate(), args[2].immediate()));
     }
@@ -991,19 +996,15 @@ pub fn generic_simd_intrinsic<'a, 'gcc, 'tcx>(
         assert_eq!(pointer_count - 1, ptr_count(element_ty0));
         assert_eq!(underlying_ty, non_ptr(element_ty0));
 
-        // The element type of the third argument must be a signed integer type of any width:
+        // The element type of the third argument must be an integer type of any width:
+        // TODO: also support unsigned integers.
         let (_, element_ty2) = arg_tys[2].simd_size_and_type(bx.tcx());
         match *element_ty2.kind() {
             ty::Int(_) => (),
             _ => {
                 require!(
                     false,
-                    InvalidMonomorphization::ThirdArgElementType {
-                        span,
-                        name,
-                        expected_element: element_ty2,
-                        third_arg: arg_tys[2]
-                    }
+                    InvalidMonomorphization::MaskWrongElementType { span, name, ty: element_ty2 }
                 );
             }
         }
@@ -1109,17 +1110,13 @@ pub fn generic_simd_intrinsic<'a, 'gcc, 'tcx>(
         assert_eq!(underlying_ty, non_ptr(element_ty0));
 
         // The element type of the third argument must be a signed integer type of any width:
+        // TODO: also support unsigned integers.
         match *element_ty2.kind() {
             ty::Int(_) => (),
             _ => {
                 require!(
                     false,
-                    InvalidMonomorphization::ThirdArgElementType {
-                        span,
-                        name,
-                        expected_element: element_ty2,
-                        third_arg: arg_tys[2]
-                    }
+                    InvalidMonomorphization::MaskWrongElementType { span, name, ty: element_ty2 }
                 );
             }
         }